1. Field of the Invention
The present invention relates to a battery control apparatus configured to estimate a state of charge of a secondary battery.
2. Description of the Related Art
A conventional technique enables estimating a state of charge (hereinafter, referred to as SOC) of a secondary battery, such as a nickel-metal hydride battery or a lithium-ion battery, and controlling a charge/discharge operation of the secondary battery based on an estimated SOC.
For example, a method discussed in Japanese Laid-Open Patent Application No. 11-174134 estimates an SOC based on a cumulatively measured charge/discharge current value of a secondary battery during a predetermined period of time.
However, immediately after starting a charge/discharge operation of a secondary battery, an estimated SOC based on a cumulative charge/discharge current value of the secondary battery is inaccurate because only a short period of time is available for measuring the charge/discharge current.
Another conventional method stores an initial value (hereinafter, referred to as “previous SOC”) that represents an SOC value immediately before a previous charge/discharge operation of a secondary battery is completed and, if the charge/discharge operation of the secondary battery resumes, estimates an SOC value based on the initial value (i.e., previous SOC).
Furthermore, a method discussed in Japanese Laid-Open Patent Application No. 2005-300418 detects a maximum voltage drop in a residual capacity of a lead-acid battery during a startup operation of a vehicle and estimates an SOC of the lead-acid battery with reference to a relationship between the maximum voltage drop and a current value corresponding to the voltage drop.
In general, the SOC of a secondary battery tends to decrease due to a self discharge that may occur when the secondary battery does not perform a charge/discharge operation. Therefore, if the secondary battery is left for a long time after a previous operation, an SOC level at which the secondary battery resumes its operation may be different from a previous SOC level. In other words, an SOC estimated from the previous SOC may not be directly used as an initial value for estimating the SOC at the time the secondary battery resumes its operation.
Furthermore, in a charge/discharge operation of a secondary battery, a polarization voltage (more specifically, electrochemical polarization voltage) may be generated due to an electromotive force of the battery although Japanese Laid-Open Patent Application No. 2005-300418 does not mention any adverse effect of the polarization voltage. Therefore, the SOC estimating method discussed in Japanese Laid-Open Patent Application No. 2005-300418 may not accurately estimate the SOC if a polarization voltage is generated.